This application relates to and incorporates herein by reference Japanese Patent Application 2001-338119 (filed on Nov. 2, 2001).
The present invention relates to printed wiring boards and methods of manufacturing printing boards, and more particularly to a printed wiring board that is a double-sided printed wiring board or a multilayer printed wiring board with a plurality of electrically-interconnected conductor pattern layers, and to the manufacturing method thereof.
In Japanese Patent Application No. 7-176846, conductor patterns on a printed wiring board are interconnected electrically by packing conductive paste, which contains metal particles and binder resin, in a via hole formed in the printed wiring board and by pressing the paste in the presence of heat. However, in that publication, interconnection between the conductor patterns is achieved by contact conduction between metal particles in the via hole and by contact conduction between the metal particles and the conductor patterns. Therefore, if the printed wiring board is used in thermally harsh conditions, such as in a vehicle, the interlayer connection resistance is likely to vary.
For example, in a high temperature environment, the reliability of the interconnection is lowered by an increase in interlayer connection resistance because the contact resistances between metal particles and between the metal particles and the conductor patterns increase due to thermal expansion of the binder resin. The problem becomes more critical as the density of the printed wiring board increases.
The present invention has been made accordingly to provide a more reliable printed wiring board.
Basically, the invention is a printed wiring board formed by interconnected layers. Each layer includes an insulator film, in which a via hole is formed. A conductor pattern located on the insulator film, and the conductor pattern includes a conductor metal. A solid conductive material is located in the via hole. The solid conductive material includes a first type of conductive material and a second type of conducting material. The first type of conducting material includes a metal, and the second type of conductive material includes an alloy formed by the metal and the conductor metal.
The invention is also a method of making a printed wiring board. The method includes packing via holes formed in an insulator material with an interlayer conducting material. The interlayer conducting material includes a first metal material and a second metal material, and the second metal material melts at a temperature higher than a predetermined temperature. The method further includes stacking layers of the insulator film with conductor patterns to form a stack such that the via holes are located between the conductor patterns in the stack. Further, the method includes forming a solid conductive material in each of the via holes to electrically connect the connector patterns by heating the stack to the predetermined temperature and pressing the stack. The solid conductive material of each via hole includes a unified conductive layer and a solid phase diffusion layer. The solid phase diffusion layer is formed by the first metal material and a conductor metal, and the conductor metal is a metal of the associated conductor pattern.